1. Field
The present invention is directed to light source anomaly state detection and, more particularly, to a light source anomaly state detection enabled to selectively control a light source based on monitored characteristics of the light source.
2. Description of the Related Art
A Wavelength Division Multiplexing (WDM) optical transmission is an example of one technique used in optical transmission systems. FIG. 9 is a block diagram showing a light source configuration for use with the WDM technique. As shown in FIG. 9, a light source 1 monitors an output optical power from a laser diode (LD) 4 using a monitoring photodiode (PD) 2 and an LD output light monitor 3, and monitors a wavelength of the output light from the LD 4 using a wavelength locker 5 and a wavelength monitor 6. When an anomaly is detected in a monitored value for the output optical power or in a monitored value for the wavelength is detected by an optical output alarm detector 7 or a wavelength alarm detector 8, the optical output is instantaneously blocked (shut down) by an optical power controller 9.
Generally, in a WDM optical transmission system, optical amplifiers which make use of EDFs (Erbium Doped Fibers) are disposed in the transmission lines to extend transmission distances. The WDM light, which is a wavelength multiplexed light composed of a plurality of signal lights with a plurality of differing wavelengths, is input to the optical amplifiers and amplified therein. When a change in the number of input wavelengths, i.e. the number of multiplexed optical signals included in the input WDM light, or some other factor causes an increase in input optical power, the amount of excitation light input to the EDF increases in Automatic Gain Control (AGC). Similarly, when a change in the number of input wavelengths or the like causes a reduction in input optical power, the amount of excitation light is reduced. Hence an amount of output optical power is controlled so as to remain constant.
FIG. 10 is a set of schematic plots illustrating optical changes in the transmission path when output from a portion of the light sources is instantaneously shut down. As shown in FIG. 10, when an anomaly in the monitored value of the output optical power is detected in a light source outputting a light of a specific wavelength, an output optical power 11 of the LD module of the light source in which the anomaly has been detected is shut down. As a result, the input optical power 12 to the EDF optical amplifier drops steeply by precisely the amount of output optical power of the shut-down light source.
Since there is a transition time (EDF excitation light emission time constant), which is a time between a time when the excitation light input to the optical amplifier and a time when emission occurs at the EDF, even though control is performed to reduce the excitation light intensity corresponding to a decrease of the input light power to the optical amplifier, there is a time when the optical amplifier remains in an excitation state corresponding to the intensity before the input optical power is reduced. This excitation state is a state having a high gain with respect to the reduced input optical power, and so an output optical power 13 from the optical amplifier temporarily increases. Thereafter, for the period (AGC circuit tracking time constant) until the automatic gain control (AGC) begins to function normally, the output optical power 13 of the optical amplifier becomes unstable, and the output optical power 14, which is the output optical power of the optical amplifier excluding the optical power of the shut-down light source, temporarily changes.
In WDM optical transmission systems, it is required that an anomaly occurring in a light source of a specific wavelength does not affect signal light of other wavelengths. Thus, even when optical output of the light source of the specific wavelength is shut down, the output optical power of the optical amplifier at other wavelengths must be maintained without any large changes.
A configuration in which an optical attenuator and an optical input monitor are provided at an optical input stage of the EDF optical amplifier is discussed in Japanese Patent Laid-Open No. H11-112435 (JP11-112435), for example. The optical attenuator allows variation based on an amount of change in the input optical power to the EDF optical amplifier. With this configuration, abrupt changes in the input optical power are absorbed by the optical attenuator, and temporary changes in the optical output power of the EDF optical amplifier are suppressed.
In the above-described and other similar configurations, such as the light source shown in FIG. 9, when an anomaly is detected in the monitored value, even if the cause is an anomaly in the various monitors and the optical output of the LD is in a controllable state, the optical output is uniformly shut down and a large level change temporarily occurs in the EDF optical amplifier in spite of the LD optical output being in a controllable state.
Further, in the above-described related art, such as the optical amplifier with the configuration disclosed in JP11-112435, it is necessary to provide the optical attenuator and the optical input monitor at an optical input stage of the optical amplifier. That causes an increase of a number of parts, requires a more complex configuration, and deterioration of noise properties.